Thermochemical process for recovering Cu from CuO or CuO2

ABSTRACT

A process for producing hydrogen comprises the step of reacting metallic Cu with Ba(OH) 2  in the presence of steam to produce hydrogen and BaCu 2  O 2 . The BaCu 2  O 2  is reacted with H 2  O to form Cu 2  O and a Ba(OH) 2  product for recycle to the initial reaction step. Cu can be obtained from the Cu 2  O product by several methods. In one embodiment the Cu 2  O is reacted with HF solution to provide CuF 2  and Cu. The CuF 2  is reacted with H 2  O to provide CuO and HF. CuO is decomposed to Cu 2  O and O 2 . The HF, Cu and Cu 2  O are recycled. In another embodiment the Cu 2  O is reacted with aqueous H 2  SO 4  solution to provide CuSO 4  solution and Cu. The CuSO 4  is decomposed to CuO and SO 3 . The CuO is decomposed to form Cu 2  O and O 2 . The SO 3  is dissolved to form H 2  SO 4 . H 2  SO 4 , Cu and Cu 2  O are recycled. In another embodiment Cu 2  O is decomposed electrolytically to Cu and O 2 . In another aspect of the invention, Cu is recovered from CuO by the steps of decomposing CuO to Cu 2  O and O 2 , reacting the Cu 2  O with aqueous HF solution to produce Cu and CuF 2 , reacting the CuF 2  with H 2  O to form CuO and HF, and recycling the CuO and HF to previous reaction steps.

BACKGROUND OF THE INVENTION

This invention was made in the course of, or under, a contract with the United States Department of Energy. It relates to the thermochemical production of hydrogen.

This is a division of application Ser. No. 934,664, filed Aug. 17, 1978 U.S. Pat. No. 4,169,884.

For a number of years hydrogen has been produced by the electrolysis of water. Electrolysis is highly inefficient due to the inefficiencies present in electricity production coupled with the efficiency of about 80% for electrolysis. The wide-spread use of hydrogen as a primary energy source will likely depend upon the efficient production of hydrogen by thermochemical processes or by electrolytic processes which have a significantly lower demand for electrical energy than that required by electrolysis of water. Thermochemical techniques involve the decomposition of water into hydrogen and oxygen through a series of chemical reactions not involving the use of fossil fuels. Preferably, the series of reactions is carried out in a closed cyclic manner, such that all products except hydrogen and oxygen are reused as reactions in the other reactions. Typical of such processes are those described in commonly assigned U.S. Pat. Nos. 3,490,871 and 4,005,184.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a chemical reaction flowsheet depicting the sequence of reactions for three embodiments of the cyclic production of hydrogen according to the process of this invention.

FIG. 2 is a chemical reaction flowsheet depicting the sequence of reaction for the recovery of Cu from CuO.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a process for producing hydrogen from copper and barium compounds.

It is a further object to provide a closed thermochemical process for producing hydrogen which requires no input other than water and thermal energy.

It is a further object to provide a hybrid process involving both thermochemical and electrolytic steps whereby the consumption of electricity is significantly reduced over that needed for the electrolysis of water. A portion of the process of this invention is also useful for recovering copper from CuO.

These and other objects are achieved according to this invention in a process for producing hydrogen comprising the step of (a) reacting Cu with Ba(OH)₂ in the presence of steam to produce hydrogen and BaCu₂ O₂. The BaCu₂ O₂ can be reacted with H₂ O to form Cu₂ O and a Ba(OH)₂ product which can be recycled to the initial reaction step. Cu can be obtained from the Cu₂ O product by several methods. In one embodiment the Cu₂ O is reacted with aqueous HF solution to provide CuF₂ and Cu. The CuF₂ is reacted with H₂ O to provide CuO and HF. CuO is decomposed is Cu₂ O and O₂. The HF and the Cu₂ O are recycled and the Cu is recovered as a product. In another embodiment Cu₂ O is reacted with aqueous H₂ SO₄ solution to provide CuSO₄ and Cu. The CuSO₄ is decomposed to form CuO and SO₃. The CuO is decomposed to form Cu₂ O and O₂. The SO₃ is recycled to form H₂ SO₄ and the Cu₂ O is recycled for reaction with H₂ SO₄. In another embodiment Cu₂ O is decomposed electrolytically to Cu and O₂. In its copper recovery aspects the process of this invention comprises recovering Cu from CuO by the steps of decomposing CuO to form Cu₂ O and O₂, reacting the Cu₂ O with aqueous HF solution to produce Cu product and CuF₂, reacting with CuF₂ with H₂ O to form CuO and Hf, and recycling the CuO and HF to previous reaction steps.

DETAILED DESCRIPTION

An aspect of this invention is the discovery of integrated chemical reaction cycles which produce H₂ or Cu in high yields without difficult separation steps. The reaction conditions for the individual reactions can be varied somewhat from the specific temperatures, pressures, times, etc. described herein and still provide the desired products. All that is required according to this invention is that the reactants, or material containing the reactants, be reacted to produce the specified products which are then separated and/or recycled to other steps in the process, regardless of the specific conditions under which the reactions are conducted.

The cyclic production of hydrogen according to the present invention proceeds according to FIG. 1. FIG. 1 illustrates two reaction steps common to each of the hydrogen production cycles and three alternative methods for regenerating the Cu reactant for the initial reaction. The first reaction is the reaction of metallic Cu with Ba(OH)₂ in the presence of steam to provide a BaCu₂ O₂ reaction product and H₂. The reaction of Cu with Ba(OH)₂ is performed by contacting Cu, preferably in the form of powder or turnings, with Ba(OH)₂ in the presence of steam at a temperature of 600-800° C. An H₂ O pressure of 1-10 atmosphere is suitable. The reaction should be carried out in copper equipment or equipment resistant to corrosion by Ba(OH)₂. The reaction can be easily carried out on a laboratory scale in a horizontal tube furnace. The Ba(OH)₂ is liquid at the reaction temperature and the copper readily reacts with the liquid to yield hydrogen as illustrated by reaction (1). ##EQU1## H₂ is carried out of the reaction zone with the steam and the steam is condensed from the gaseous phase, providing a gaseous H₂ product. Since Cu and Ba(OH)₂ are obtained upon treatment of the BaCu₂ O₂ product in subsequent steps in the cycle, it is not necessary at this point to separate unreacted Cu and Ba(OH)₂ from the solid BaCu₂ O₂ containing product.

In the second step in the cycle, the solid product of reaction (1) is reacted with H₂ O to provide Ba(OH)₂ and Cu₂ O according to the following reaction:

    BaCu.sub.2 O.sub.2 +H.sub.2 O →Ba(OH).sub.2 +Cu.sub.2 O (2)

This reaction is easily conducted by contacting the solid product of reaction (1) with water at about 25-100° C. at atmosphere pressure. Since the Ba(OH)₂ product is partially water soluble, the reaction and separation can be performed simultaneously be repeated boilings and filtration of the solid product of reaction (1), to provide a Ba(OH)₂ solution and a solid product containing Cu₂ O. An inert atmosphere, e.g. N₂ or Ar, should be used to prevent formation of BaCO₃ from CO₂ present in air. In the laboratory, this reaction is easily conducted in a Soxhlet apparatus with the solid reactant on a glass frit and H₂ O in the reservoir. The H₂ O is heated to cause boiling or evaporation and subsequent condesation. The condensate contacts the solid reactant causing the formation of Ba(OH)₂ which dissolves in the excess condensed water. The Ba(OH)₂ solution is evaporated and the solid Ba(OH)₂ is recycled to reaction (1). Since the Cu-BaO-steam system is essentially equivalent to the Cu-Ba(OH)₂ -steam system, it will be apparent that BaO can be used in reaction (1) instead of Ba(OH)₂. If BaO is to be recycled, the Ba(OH)₂ product of reaction (2) can be heated, e.g. to 900° C., to decompose the Ba(OH)₂ to BaO prior to recycle.

Cu can be recovered from the Cu₂ O product of reaction (2) in several ways. In one embodiment (represented by the solid lines in FIG. 1) Cu₂ O is reacted with aqueous HF solution according to the following reaction:

    2Cu.sub.2 O+4HF(aq)→2CuF.sub.2 +2Cu+2H.sub.2 O      (3a)

This reaction can be performed by contacting Cu₂ O with an aqueous HF solution (15 to 48 wt. %) at a temperature of 25 to 80° C. The reaction goes essentially to completion. The reaction product mixture is an aqueous CuF₂ solution containing metallic copper which can be separated by filtration, sedimentation, etc. The separated Cu is recycled to reaction (1). To facilitate Cu separation, sufficient water should be present in the HF solution to maintain the CuF₂ product in solution. CuF₂ is recovered from solution by evaporation and the resulting CuF₂ powder is reacted with steam at temperatures of 250-500° C. by the following reaction:

    CuF.sub.2 +2H.sub.2 O→2CuO+4HF(g)                   (4a)

This reaction is conducted by passing excess steam either over or through the CuF₂ powder. The H₂ O pressure is not critical, with 1-10 atm. being suitable. The reaction proceeds essentially to completion. HF is recovered from the reaction mixture by condensing the gaseous product containing excess steam to provide an aqueous HF solution. The aqueous HF solution is adjusted to the appropriate concentration for recycle to reaction (3a). CuO which remains as a solid product, is thermally decomposed according to the following reaction: ##EQU2## This reaction is carried out by heating the solid product of reaction (4a) to about 950 ° to 1050° C. in the presence of steam. The preferred H₂ O pressure is about 1 atm. The presence of steam significantly lowers the reaction temperature and produces Cu₂ O particles rather than a frozen liquid. Excess H₂ O can be condensed from the reaction atmosphere leaving a gaseous O₂ product. The solid Cu₂ O is recycled to reaction (3a) for treatment with HF.

In a second embodiment (represented by dashed lines in FIG. 1) Cu₂ O from reaction 2 is reacted with aqueous sulfuric acid according to the following reaction:

    2Cu.sub.2 O+2H.sub.2 SO.sub.4 (aq)→2CuSO.sub.4 +2Cu+2H.sub.2 O (3b)

This reaction is carried out by contacting the solid product of reaction (2) with aqueous H₂ SO₄ solution (about 20 to 60 wt.%) at a temperature of 25° to 80° C. The reaction goes essentially to completion. The CuSO₄ product is recovered in the form of an aqueous solution. The Cu precipitates and can be separated by filtration, sedimentation, etc. for recycle to reaction (1). The H₂ SO₄ solution should contain sufficient H₂ O to keep the resulting CuSO₄ in solution thereby facilitating recovery of Cu precipitate. The CuSO₄ product is recovered from solution by evaporation and thermally decomposed to CuO by heating to 600°-900° C., according to the following reaction:

    2CuSO.sub.4 →2CuO+2SO.sub.3                         (4b)

to yield solid CuO and SO₃. The reaction can be conducted in air and SO₃ can be recovered from the reaction mixture by passage through an aqueous solution. The CuO product is decomposed by heating at from 950° to 1050° C. in steam atmosphere at about 1 atmosphere H₂ O pressure according to reaction (5a) described earlier. The Cu₂ O product is recycled to reaction (3b). The SO₃ product of reaction (4b) is dissolved in water to provide H₂ SO₄ solution by the following reaction:

    SO.sub.3 +H.sub.2 O→H.sub.2 SO.sub.4                (5b)

The H₂ SO₄ is recycled to reaction (3b). The preferred method of recovering SO₃ is to bubble the gaseous product through a concentrated H₂ SO₄ solution, e.g. 37 M, to form oleum, which can then be diluted with water to be appropriate concentration for reaction (3b).

In a third embodiment (represented by dotted lines in FIG. 1) Cu is recovered from the Cu₂ O product of reaction (2) by electrolysis according to the following reaction: ##EQU3## The electrical energy necessary for this step is about 60% less than that required for direct electrolysis of water. The electrolysis can be performed in molten Cu₂ O or at a temperature below the Cu₂ O melting point in a molten salt solution. Suitable electrodes are a Cu cathode and an anode of a metal more noble than Cu, e.g. Pt, for evolving O₂. Those skilled in the art of electrochemistry can carry out the electrolysis according to the well-established principles for carrying out electrolysis from melts, such as those described for molten salt mixtures and molten salt mixtures containing dissolved Al₂ O₃ in Potter, Electrochemistry, Principles and Aplications, Cleaver-Hume Press Ltd., London, (1956) pp. 316-327, which is incorporated herein by reference.

The following examples demonstrate several of the reactions of the present invention.

EXAMPLE I

Reaction 1 was carried out in a copper boat or in a single crystal MgO crucible positioned in a silica tube within a horizontal furnace. The boat contained 1.6 grams Cu turnings and 5 grams Ba(OH)₂. Steam at 500 ml water per hour was preheated and introduced into the reactor. The steam was mixed with argon at about 100 cc/min. to facilitate subsequent H₂ analysis of the product gas stream. The exiting gases were passed through a water-cooled condenser followed by a drying column and a hydrogen monitoring assembly. Upon heatup and Ba(OH)₂ melted. The evolution of hydrogen started at about 875° K. (692° C.) and reached a maximum at about 1070°±20° K. The reaction product, BaCu₂ O₂, was recovered from the boat and identified by x-ray diffraction and chemical analysis. In the absence of steam the yield of hydrogen was about 18% while in the presence of steam the yield of hydrogen was about 100% based upon the oxidation to Cu⁺. Steam prevents the decomposition of Ba(OH)₂ and thereby provides the source for H₂ production.

EXAMPLE II

The solid product of Example I was reacted with H₂ O in a Soxhlet extractor at about 345° K. H₂ O in the reservoir of the apparatus was evaporated and condensed over the solid BaCu₂ O₂ whereupon the Ba(OH)₂ was formed and dissolved. The resulting Ba(OH)₂ solution was collected in the reservoir. After reaction ceased, the residue remaining was identified as Cu₂ O.

EXAMPLE III

A known amount of Cu₂ O was dissolved in an aqueous HF solution (about 20 wt.%) in a polytetrafluoroethylene beaker with magnetic stirring. After about 30 minutes, finely divided Cu was separated by filtration from the blue CuF₂ solution. The yield of Cu as 86% of the stoichiometric amount.

EXAMPLE IV

The reaction of CuF₂ and H₂ O is performed by heating the CuF₂ powder in a HF-resistant container, i.e. polytetrafluoroethylene in a steam atmosphere (about 1 atm. pressure) at a temperature of about 250° C. HF solution is recovered as a condensate, leaving a solid CuO product. The recovered HF solution is adjusted to the appropriate concentration for use in reaction (3a).

EXAMPLE V

CuO was heated in a stream of steam containing argon. About 100% of the oxygen produced according to reaction (5a) was evolved upon heating to 1000° C. Under similar conditions in argon atmosphere, only about 3-4% of the oxygen was evolved upon heating to 1000° C. and less than 65% had been evolved after heating to nearly 1200° C.

Another aspect of this invention is the recovery of Cu from CuO as illustrated in FIG. 2. The reactions are conducted in the same manner as described above. CuO, obtained for example from scrap material or naturally occurring ores such as tenorite and paramelaconite, is decomposed, preferably in the presence of steam according to reaction (5a). The CuO is heated at from 950-1050° C. in steam, e.g. at 1 atm. H₂ O pressure, it provide Cu₂ O and O₂. The solid Cu₂ O product is reacted according to reaction (3a) with aqueous 15-48 wt.% HF solution at 25-80 ° C. to provide a CuF₂ solution and a metallic Cu precipitate which is recoverably by filtration, sedimentation, etc. The HF solution should contain sufficient H₂ O to keep the resulting CuF₂ in solution, thereby facilitating recovery of Cu precipitate. The CuF₂ is recovered from solution by evaporation and reacted with steam at 250-500° C. according to reaction (4a) to provide solid CuO and gaseous HF. The HF is easily recovered from the gas phase by condensation into water to provide an HF solution. The HF concentration of the solution is adjusted for recycle to reaction and solid CuO is recycled to reaction (5a) for further treatment.

It can be seen that Cu can also be recovered from Cu₂ O such as naturally occurring cuprite ores by the same reaction cycle. Rather than adding CuO to reaction (5a), Cu₂ O is added to reaction (3a) as indicated by the dashed line FIG. 2 and the sequence proceeds by reactions 3a, 4a, and 5a, respectively. By appropriate mass balance, mixtures of Cu₂ O and CuO can be processed according to FIG. 2.

It will be apparent to those skilled in the art that all separations in the processes of this invention need not be complete and all reagents need not be directly recycled. For example, fresh H₂ O can be added to any step where it is needed and can be allowed to escape as steam or used to provide heat for other portions of the process. Other products such as HF or H₂ SO₄ may be diverted to other uses as economics might dictate, and be replaced with fresh material. These and other such obvious modifications are contempleted as equivalents of the processes described herein. 

What is claimed is:
 1. A cyclic thermochemical process for recovering Cu from CuO or Cu₂ O comprising the steps of(a) in a heating zone, heating CuO to sufficient temperature to form Cu₂ O and O₂. (b) withdrawing the O₂ so formed from said zone as a gas stream, (c) reacting Cu₂ O with aqueous HF solution to form Cu and CuF₂, (d) selectively recovering Cu formed in step (c), (e) reacting CuF₂ with H₂ O to form CuO and HF, (f) recycling CuO from step (e) to step (a), and(g) recycling HF from step (e) to step (c).
 2. The process of claim 1 wherein said temperature is in the range of 950-1050° C.
 3. The process of claim 1 wherein said step (c) is conducted at a temperature in the range of from 25-80° C.
 4. A cyclic thermochemical process for recovering Cu from CuO or Cu₂ O comprising the steps of:(a) heating CuO to a temperature in the range of from about 950-1050° C. in the presence of steam to form Cu₂ O and O₂, (b) reacting Cu₂ O with aqueous HF solution to form Cu and CuF₂, (c) selectively recovering Cu formed in step (b), (d) reacting CuF₂ with H₂ O to form CuO and HF, (e) recycling CuO from step (d) to step (a), and (f) recycling HF from step (d) to step (b).
 5. The process of claim 4 wherein said steam is in admixture with argon.
 6. The process of claim 4 wherein said step (b) is conducted at a temperature in the range of 25° to 80° C.
 7. A cyclic thermochemicl process for recovering Cu from CuO or Cu₂ O comprising the steps of(a) in a heating zone, heating CuO to a temperature in the range of from about 950° to 1050° C. in the presence of steam to form Cu₂ O, and O₂, (b) withdrawing the O₂ so formed from said zone as a gas stream, (c) reacting Cu₂ O formed in step (a) with aqueous HF solution to form Cu and CuF₂. (d) selectively recovering Cu formed in step (c), (e) reacting CuF₂ formed in step (c) with H₂ O to form CuO and HF, (f) recycling CuO from step (e) to step (a), and (g) recycling HF from step (e) to step (c). 